Method and apparatus for trimming current limit and frequency to maintain a constant maximum power

ABSTRACT

Methods and apparatuses to trim current limit (I p ) and frequency (f) to maintain a constant maximum power. In one aspect of the invention, a power supply regulator having a peak current limit detection threshold and an operating frequency is disclosed. The peak current limit threshold or the operating frequency are adjusted specifically to maintain the I p   m ·f n  product substantially constant from one power supply controller to another where m is substantially equal to 2 and n is substantially equal to 1. In another aspect of the invention, the power supply regulator has a control circuit having a control threshold current. The peak current limit threshold or the operating frequency are adjusted specifically to maintain the product of I p   m ·f n  divided by the control threshold current substantially constant from one power supply controller to another where m is substantially equal to 2 and n is substantially equal to 1.

RELATED APPLICATION

[0001] This application claims priority to U.S. provisional applicationserial No. 60/316,583, filed Aug. 31, 2001, entitled “Method AndApparatus For Trimming Current Limit And Frequency To Maintain AConstant Maximum Power.”

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to power supplies and,more specifically, the present invention relates to trimming a powersupply for increased accuracy in the maximum deliverable power of thepower supply.

[0004] 2. Background Information

[0005] Most battery operated portable electronic products such as cellphones, personal digital assistants (PDAs), etc., require a low poweralternating current (AC) to direct current (DC) charger power supplywith a constant voltage and constant current (CC/CV) characteristics forcharging batteries. In known power supplies, the peak current limitthreshold and operating frequency are trimmed independently duringmanufacturing. The two parameters have their own specifications withoutany relationship to each other. Other known power supplies employdiscrete solutions where the tolerances of components like resistors,and capacitors will affect the peak current limit threshold, and theoperating frequency, and by doing so also affecting the maximumdelivered power.

SUMMARY OF THE INVENTION

[0006] Methods and apparatuses for trimming a power supply aredisclosed. In one aspect of the invention, a power supply regulator hasa peak current limit detection threshold and an operating frequency. Inthe one embodiment, either the peak current limit threshold or theoperating frequency are adjusted specifically to maintain the product ofthe square of the peak current limit and the operating frequencysubstantially constant from one power supply controller to another. Inone embodiment, the power supply regulator is used to implement aflyback converter. In another embodiment, the power supply regulator isused to implement a buck converter. In one embodiment, the power supplyregulator operates with a substantially constant frequency under alloperating conditions. In another embodiment, the power supply regulatoroperates with a substantially constant frequency under a fixed range ofoperating conditions.

[0007] In another aspect of the present invention, a power supplyregulator has a peak current limit detection threshold and an operatingfrequency. The peak current limit threshold and/or the operatingfrequency are adjusted during the manufacture of the power supplycontroller to maintain the product of the square of the peak currentlimit and the operating frequency substantially constant from one powersupply regulator to another. In one embodiment, the power supplyregulator is used to implement a flyback converter. In anotherembodiment, the power supply regulator is used to implement a buckconverter. In one embodiment, the power supply regulator operates with asubstantially constant frequency under all operating conditions. Inanother embodiment, the power supply regulator operates with asubstantially constant frequency under a fixed range of operatingconditions

[0008] In yet another aspect of the present invention, a switched modepower supply includes a power supply input and a power supply output. Apower supply regulator is coupled to the power supply input and anenergy transfer element input. The energy transfer element is furthercoupled to the power supply output. The power supply regulator has anoperating frequency and a threshold to detect the peak energy transferelement input current. In one embodiment, the operating frequency and/orthe peak current detection threshold of the power supply regulator areadjusted to maintain the product of the square of the peak current limitand the operating frequency substantially constant from one power supplyregulator to another. In one embodiment, the switched mode power supplyis a flyback converter. In another embodiment, the switched mode powersupply is a buck converter. In one embodiment, the power supplyregulator operates with a substantially constant frequency under alloperating conditions. In another embodiment, the power supply regulatoroperates with a substantially constant frequency under a fixed range ofoperating conditions.

[0009] In still another aspect of the present invention, a switched modepower supply includes a power supply input and a power supply output. Apower supply regulator is coupled to the power supply input and anenergy transfer element input. In one embodiment, the power supplyregulator has a fixed operating frequency when the switched mode powersupply is providing substantially maximum output power. In oneembodiment, the power supply regulator has a threshold to detect thepeak energy transfer element current. In one embodiment, the operatingfrequency and/or the peak current detection threshold of the powersupply regulator are adjusted during the manufacture of the power supplyregulator to maintain the product of the square of the peak currentlimit and the operating frequency substantially constant from one powersupply regulator to another. In one embodiment, the switched mode powersupply is a flyback converter. In another embodiment, the switched modepower supply is a buck converter.

[0010] Additional features and benefits of the present invention willbecome apparent from the detailed description and figures set forthbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention detailed is illustrated by way of exampleand not limitation in the accompanying figures.

[0012]FIG. 1 is a block diagram illustrating one embodiment of a powersupply regulator in accordance with the teachings of the presentinvention.

[0013]FIG. 2 is a diagram illustrating the relationship between the dutycycle and the current (I_(C)) into the control terminal of oneembodiment of a power supply regulator in accordance with the teachingsof the present invention.

[0014]FIG. 3 is a diagram illustrating the relationship between the peakcurrent limit threshold I_(p) and the current (I_(C)) into the controlterminal of another embodiment of a power supply regulator in accordancewith the teachings of the present invention.

[0015]FIG. 4 is a schematic diagram illustrating one embodiment of apower supply that has a relatively constant voltage and currentcharacteristic in accordance with the teachings of the presentinvention.

[0016]FIG. 5 a diagram illustrating the relationship between the outputcurrent and output voltage of one embodiment of a power supply inaccordance with the teachings of the present invention.

[0017]FIG. 6 is a schematic diagram illustrating another embodiment of apower supply that has a relatively constant voltage and currentcharacteristic in accordance with the teachings of the present invention

[0018]FIG. 7 is a schematic of one embodiment of a trim circuit blockused for frequency trim or peak current limit threshold trim inaccordance with the teachings of the present invention.

DETAILED DESCRIPTION

[0019] Embodiments of methods and apparatuses for trimming a powersupply for increased accuracy in the maximum deliverable power of thepower supply are disclosed. In the following description, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone having ordinary skill in the art that the specific detail need notbe employed to practice the present invention. In other instances,well-known materials or methods have not been described in detail inorder to avoid obscuring the present invention.

[0020] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures or characteristics may be combined in any suitable manner inone or more embodiments.

[0021] In various embodiments, low cost integrated solutions areprovided for controlling the maximum deliverable power by trimming ofthe internally set peak current limit threshold I_(p), and operatingfrequency f of a power supply regulator in such a way that the productof the oscillating frequency f and the square of the peak current limitthreshold I_(p) is a constant or substantially uniform from oneregulator to the next. Rather than defining an independent operatingfrequency f, and an independent peak current limit threshold I_(p), thetwo are made dependent on each other, and will together determine themaximum deliverable power in an AC to DC or DC to DC power supply.

[0022] In one embodiment, the maximum deliverable power in a switch modepower supply that is designed to work in discontinuous operation isdetermined by the primary inductance of the transformer, operatingfrequency f, and peak current limit threshold I_(p). In one embodiment,to keep the maximum deliverable power substantially constant, theoperating frequency f and the peak current limit threshold I_(p) aretrimmed in a way such that the product of Equation 1 below:

I _(p) ^(m) ·f ^(n)  (Equation 1)

[0023] is substantially constant. In one embodiment, m is substantiallyequal to 2 and n is substantially equal to 1 such that the product ofthe square of the peak current limit threshold I_(p) and the operatingfrequency f is constant. The operating frequency f and the peak currentlimit threshold I_(p) can be trimmed during manufacturing. Variousembodiments of the present invention can be used to provide a relativelyaccurate, constant voltage and current characteristic for batterychargers.

[0024] To illustrate, FIG. 1 is a block diagram illustrating oneembodiment of a power supply regulator 321 in accordance with theteachings of the present invention. In one embodiment, the power supplyregulator 321 is included on a single monolithic chip having as few asthree electrical terminals. The power supply regulator 321 includes apower switch 365 coupled between drain terminal 323 and source terminal329. In one embodiment, power switch 365 includes a metal oxidesemiconductor field effect transistor (MOSFET).

[0025] As shown in the embodiment depicted in FIG. 1, the power supplyregulator 321 also includes a control current sensor 369 coupled tosense the excess control current through the control terminal 325. Theexcess control current is the total control terminal current less thesupply current of the regulator. The supply current of the regulator isknown. Therefore, the total Control terminal current can be deduced fromthe excess control current. The current received through the controlterminal 325 is responsive to a reflected voltage from an energytransfer element of a power supply in FIG. 4 that the power supplyregulator 321 is coupled to regulate.

[0026] In one embodiment, the power switch 365 is switched in responseto the current received through the control terminal 325. A controlcircuit 367 is coupled to control current sensor 369 and power switch365. In one embodiment, the control circuit 367 modulates the duty cycleand the peak current limit threshold I_(p) of the power switch 365 toregulate the output of the power supply in FIG. 4. Accordingly, powerswitch is switched on and off by control circuitry 367 to enable anddisable the delivery of power to the output of the power supply toregulate the power supply output. In so doing, the control circuit 367modulates the duty cycle of the power switch 365 when the Controlterminal current exceeds a control threshold current. In one embodiment,the control circuit 367 reduces the duty cycle when the Control terminalcurrent exeeds the control threshold current. In one embodiment, thecontrol circuit 367 reduces the peak current limit threshold I_(p) whenthe Control terminal current is below the same control thresholdcurrent.

[0027] In one embodiment, the current limit comparator 368 compares thecurrent of the switch 365 to a threshold. When the current of the switch365 exceeds the threshold of the comparator, the switch is turned offimmediately for the remainder of the switching cycle. In one embodiment,the peak current limit threshold trim block 370 sets the peak currentlimit threshold I_(p) of the current limit comparator at the controlthreshold current. The peak current limit threshold I_(p) is reducedwhen the Control terminal current goes below the control thresholdcurrent. In one embodiment, the power supply regulator 321 also includesa start-up circuit 371 coupled to control terminal 325, drain terminal323 and control circuit 367. The power supply regulator 321 alsoincludes an oscillator circuit 373, and a frequency trim circuit 372 toprogram the operating frequency in the oscillator circuit 373. In oneembodiment, oscillator 373 is coupled to provide an oscillating signalhaving the operating frequency f, which is received by control circuit367 to switch power switch 365. In one embodiment, power switch 365 isswitched in response to the oscillating signal received from oscillator373.

[0028]FIG. 2 shows a diagram illustrating the relationship between theduty cycle and the current (I_(C)) into the control terminal of oneembodiment of power supply regulator 321. The diagram shows twodifferent regions of operation. In the first region, the current (I_(C))into the control terminal is lower than the Control threshold currentand the duty cycle is maximum, and the power supply regulator 321operates in current limit mode. In this mode, the peak current limitthreshold I_(p) is reduced to regulate the current at the output of thepower supply. In the second region with increased current (I_(C)) intothe control terminal output voltage is controlled by regulating the dutycycle.

[0029]FIG. 3 shows a diagram illustrating the relationship between thepeak current limit threshold I_(p) and the current (I_(C)) into thecontrol terminal of one embodiment of power supply regulator 321. Thepeak current limit threshold I_(p) increases with increased current(I_(C)) into the control terminal of the power supply regulator 321.

[0030]FIG. 4 shows one embodiment of a flyback power supply that has arelatively constant voltage and current characteristic. The feedbackinformation is provided to the power supply regulator 321 at its controlterminal. The current at the control terminal is proportional to thevoltage across resistor 235, which in turn is responsive to the outputvoltage 200. The power supply regulator 321 reduces the duty cycle ofthe integrated power metal oxide semiconductor field effect transistor(MOSFET) included in power supply regulator 321 when the voltage acrossresistor 235 increases above the voltage necessary to establish aControl threshold current. In this section, the output is in constantvoltage mode. The power supply regulator 321 reduces the peak currentlimit threshold I_(p) of the integrated power MOSFET when the voltageacross resistor 235 decreases below the voltage necessary to establishthe Control threshold current. The peak current limit threshold I_(p) isreduced as a function of the voltage across resistor 235 to regulate theoutput load current.

[0031]FIG. 5 is a diagram illustrating the relationship between theoutput current and output voltage of one embodiment of a power supply inaccordance with the teachings of the present invention. As can be seenin curve 400, the power supply exhibits a substantially constant outputcurrent and voltage characteristic. That is, as output currentincreases, the output voltage remains substantially constant until theoutput current reaches an output current threshold. As the outputcurrent approaches the output current threshold, the output voltagedecreases as the output current remains substantially constant over thedrop in output voltage. It is appreciated that the constant outputvoltage and constant output current characteristics of the presentinvention are suitable for battery charger applications or the like.

[0032]FIG. 6 shows one embodiment of a buck converter power supply 601that has a relatively constant voltage and current characteristic. Thefeedback information is provided to the power supply regulator 321 atits control terminal. The current at the control terminal isproportional to the voltage across resistor 603, which in turn isresponsive to the DC output 600 voltage. The power supply regulator 321reduces the duty cycle of the integrated power metal oxide semiconductorfield effect transistor (MOSFET) included in power supply regulator 321when the voltage across resistor 603 increases above the voltagenecessary to establish the Control threshold current. In this section,the output is in constant voltage mode. The power supply regulator 321reduces the peak current limit threshold I_(p) of the integrated powerMOSFET when the voltage across resistor 603 decreases below the voltagenecessary to establish the Control threshold current. The peak currentlimit threshold I_(p) is reduced as a function of the voltage acrossresistor 603 to regulate the output load current.

[0033]FIG. 7 is a schematic of one embodiment of a trim circuit blockused for frequency trim or peak current limit threshold I_(p) trim inaccordance with the teachings of the present invention. In this circuit,I_(ref) is a reference current source, and I_(trim) is the trimmedcurrent source. I_(trim) is a current mirror of I_(ref). The ratio ofthe current mirror changes as more zeners are trimmed. For example whenzener 750 is not trimmed, transistor 730 will be practically open sincethe voltage drop of the zener will inhibit any conduction through thetransistor. When zener 750 is trimmed (shorted), transistor 730 willconduct and be in parallel to transistor 720, thereby changing the ratioof the current mirror.

[0034] For the oscillator frequency block, I_(trim) can be used tocharge and discharge a capacitor. For the current limit comparator,I_(trim) can be used to set the threshold of the comparator. In oneembodiment, the peak current limit threshold I_(p) can be trimmed first.Then the target frequency or operating frequency f can be adjusted bythe measured current limit so that the product of the peak current limitthreshold I_(p) squared and operating frequency f will remain constant.This method will provide a constant deliverable power to the output loadof the power supply.

[0035] In yet another embodiment, the operating frequency f can beadjusted based on the peak current limit threshold I_(p) and controlthreshold current so that the product of I_(p) ^(m)·f^(n) divided by thecontrol threshold current is constant. Since the control thresholdcurrent tracks the output voltage of the power supply, this methodessentially compensates the deliverable power for the output voltage,resulting in constant power supply output current at the maximum outputpower point on the power supply output characteristic.

[0036] In the foregoing detailed description, the method and apparatusof the present invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. A power supply regulator, comprising: a powerswitch coupled between first and second terminals; a control circuitcoupled to switch the power switch; a current limit circuit coupled tothe power switch and the control circuit to control a current throughthe power switch, the current limit circuit having a peak current limitthreshold (I_(p)); and an oscillator coupled to provide the controlcircuit an oscillating signal, the control circuit to switch the powerswitch in response to the oscillating signal, the oscillating signalhaving an operating frequency (f), wherein at least one of the peakcurrent limit threshold (I_(p)) and the operating frequency (f) areadjusted to maintain a product of I_(p) ^(m)·f^(n) substantiallyconstant.
 2. The power supply regulator of claim 1 wherein where m issubstantially equal to 2 and n is substantially equal to
 1. 3. The powersupply regulator of claim 1 wherein the product of I_(p) ^(m)·f^(n) ofthe power supply regulator is substantially constant with a product ofI_(p) ^(m)·f^(n) of another power supply regulator.
 4. The power supplyregulator of claim 1 wherein said at least one of the peak current limitthreshold (I_(p)) and the operating frequency (f) are adjusted duringmanufacture of the power supply regulator.
 5. The power supply regulatorof claim 1, wherein the power supply regulator is part of a flybackconverter power supply.
 6. The power supply regulator of claim 1 whereinthe power supply regulator is part of a buck converter power supply. 7.The power supply regulator of claim 1 wherein the operating frequency(f) is substantially constant during operation under all operatingconditions of the power supply regulator.
 8. The power supply regulatorof claim 1 wherein the operating frequency (f) is substantially constantduring operation under a fixed range of operating conditions of thepower supply regulator.
 9. A switched mode power supply, comprising: anenergy transfer element having an output coupled to an output of theswitched mode power supply; and a power supply regulator coupled to aninput of the switched mode power supply and an input of the energytransfer element, the power supply regulator coupled to operate at anoperating frequency (f) and coupled to detect a current flowing throughthe input of the energy transfer element substantially equal to a peakcurrent limit detection threshold (I_(p)), wherein at least one of thepeak current limit detection threshold (I_(p)) and the operatingfrequency (f) are adjusted to maintain a product of I_(p) ^(m)·f^(n)substantially constant.
 10. The switched mode power supply of claim 9wherein where m is substantially equal to 2 and n is substantially equalto
 1. 11. The switched mode power supply of claim 9 wherein the productof I_(p) ^(m)·f^(n) of the switched mode power supply is substantiallyconstant with a product of I_(p) ^(m)·f^(n) of another switched modepower supply.
 12. The switched mode power supply of claim 9 wherein saidat least one of the peak current limit threshold (I_(p)) and theoperating frequency (f) are adjusted during manufacture of the switchedmode power supply.
 13. The switched mode power supply of claim 9 whereinthe switched mode power supply is a flyback converter.
 14. The switchedmode power supply of claim 9 wherein the switched mode power supply is abuck converter.
 15. The switched mode power supply of claim 9 whereinthe operating frequency (f) of the switched mode power supply issubstantially constant during operation under all operating conditionsof the switched mode power supply.
 16. The switched mode power supply ofclaim 8 wherein the operating frequency (f) of the switched mode powersupply is substantially constant during operation under a fixed range ofoperating conditions of the switched mode power supply.
 17. A method,comprising: switching a power switch of a power supply regulator at anoperating frequency (f); measuring a current through the power switchand maintaining the current through the power switch below a peakcurrent limit threshold (I_(p)); and adjusting at least one of the peakcurrent limit threshold (I_(p)) and the operating frequency (f) tomaintain a product of I_(p) ^(m)·f^(n) substantially constant.
 18. Themethod of claim 17 wherein where m is substantially equal to 2 and n issubstantially equal to
 1. 19. The method of claim 17 wherein the productof I_(p) ^(m)·f^(n) of the power supply regulator is substantiallyconstant with a product of I_(p) ^(m)·f^(n) of another power supplyregulator.
 20. The method of claim 17 wherein said adjusting at leastone of the peak current limit threshold (I_(p)) and the operatingfrequency (f) occurs during manufacture of the power supply regulator.21. The method of claim 17 wherein the power supply regulator isincluded in a flyback converter power supply.
 22. The method of claim 17wherein the power supply regulator is included in a buck converter powersupply.
 23. The method of claim 17 wherein the operating frequency (f)is substantially constant during operation under all operatingconditions of the power supply regulator.
 24. The method of claim 17wherein the operating frequency (f) is substantially constant duringoperation under a fixed range of operating conditions of the powersupply regulator.
 25. A power supply regulator, comprising: a powerswitch coupled between first and second terminals; a control circuitcoupled to switch the power switch, the control circuit having a controlthreshold current, above which the control circuit is adapted to reducea duty cycle at which the power switch is switched; a current limitcircuit coupled to the power switch and the control circuit to control acurrent through the power switch, the current limit circuit having apeak current limit threshold (I_(p)); and an oscillator coupled toprovide the control circuit an oscillating signal, the control circuitto switch the power switch in response to the oscillating signal, theoscillating signal having an operating frequency (f), wherein at leastone of the peak current limit threshold (I_(p)) and the operatingfrequency (f) are adjusted to maintain a product of I_(p) ^(m)·f^(n)divided by the control threshold current substantially constant.
 26. Thepower supply regulator of claim 25 wherein where m is substantiallyequal to 2 and n is substantially equal to
 1. 27. The power supplyregulator of claim 25 wherein the product of I_(p) ^(m)·f^(n) divided bythe control threshold current of the power supply regulator issubstantially constant with a product of I_(p) ^(m)·f^(n) divided by acontrol threshold current of another power supply regulator.
 28. Thepower supply regulator of claim 25 wherein said at least one of the peakcurrent limit threshold (I_(p)) and the operating frequency (f) areadjusted during manufacture of the power supply regulator.
 29. The powersupply regulator of claim 25, wherein the power supply regulator is partof a flyback converter power supply.
 30. The power supply regulator ofclaim 25 wherein the power supply regulator is part of a buck converterpower supply.
 31. The power supply regulator of claim 25 wherein theoperating frequency (f) is substantially constant during operation underall operating conditions of the power supply regulator.
 32. The powersupply regulator of claim 25 wherein the operating frequency (f) issubstantially constant during operation under a fixed range of operatingconditions of the power supply regulator.